Numerous helicopter rotors are provided with hinged blades, either on ball bearings or else on elastomer bearings. When stationary, with no centrifugal force urging the blades into a “flat” configuration, it can happen that the blades of such rotors are subjected to excessive vertical movements (flapping), leading to abnormal stresses in the elements of the rotor and possibly damaging them severely. This phenomenon can arise in particular under the following circumstances:                gusts of wind;        motion of a ship having the helicopter on board; and        folding the blades, reversing the static moment of the blades when their centers of gravity lie between the flapping hinge axis and the axis of the rotor.        
For these reasons, hinged rotor hubs are generally fitted with low abutments and with high abutments that serve to limit the amplitude of such flapping movements, respectively downwards and upwards.
A particular object of the present invention is to propose an improvement to the rotors described in French patent No. 2 725 687 and U.S. Pat. No. 5,588,801.
Those patents themselves seek to simplify the devices for limiting blade flapping and comprising independent top abutments for each blade; the device described in those patents comprises high abutments projecting from a common ring that is coaxial with the hub; the ring is mounted to turn about a rotor axis on a top portion of the hub; the device comprises flyweights connected to the ring via drive means for turning the ring between a “ground” position in which the high abutments are positioned facing respective portions of the blade in order to be active, and a “flight” position in which the high abutments are retracted out of the range of said portions of the blade; those drive means make use of the centrifugal force applied to the flyweights for retracting the high abutments, and include return springs for returning the ring to the “ground” position; those means oppose each other, with centrifugal force overcoming the return force once the speed of rotation of the rotor exceeds a predetermined value.
The device described in those patents includes an abutment serving to avoid untimely retraction of the ring carrying the blade abutments while on the ground, but for the ring turning in one direction only; if the ring turns in the opposite direction, such untimely retraction becomes possible once sufficient force is applied to overcome the return forces of the springs; although the springs are comfortably dimensioned, it has nevertheless been found that this precaution can be insufficient, particularly prior to a flight, while the pilot is verifying that the servo-controls that act on the rotor blades (the cyclic pitch and the collective pitch) are operating properly.
It has been found that when the floatingly-mounted low-abutment droop-restrainer ring is in a center position, all of the blades have a small amount of freedom to move vertically (through about 1.5°); however, when the low-abutment drop-restrainer ring is off-center, some of the blades can reach their high abutments while the opposite blades reach their low abutments.
If pitch variation is then applied by a servo-control to a blade in the high abutment position, then the displacement of the abutment shoe of the blade will rub against the retractable ring and might turn it sufficiently to completely unlock all of the high abutments in undesired manner.